The research described in this proposal is directed towards understanding the mechanism of asparagine- linked glycosylation of proteins in the endoplasmic reticulum (ER) by the oligosaccharyltransferase (OST). Particular emphasis will be placed upon (i) determining whether the STT3B isoform of the OST performs a general role in posttranslational glycosylation of sites that are skipped by the STT3A isoform of the OST, (ii) elucidating how the two OST isoforms contribute to hypoglycosylation of proteins in cells with defects in the assembly of the dolichol-linked oligosaccharide donor for N-linked glycosylation, and (iii) obtaining a cryo-electron microscopy structure of the OST in a complex with a translating ribosome and a protein translocation channel. Current evidence indicates that the STT3A isoform of the OST is primarily responsible for cotranslational N-glycosylation of nascent polypeptides as they pass through the protein translocation channel. The STT3B isoform can mediate posttranslational glycosylation of unfolded proteins. Novel insight into the in vivo role of the STT3B isoform of the OST will be obtained by identifying additional classes of STT3B substrates. Integral membrane proteins with glycosylation sites near the membrane bilayer, proteins with carboxyl-terminal glycosylation sites or multiple closely spaced glycosylation sites will be tested as potential STT3B substrates. Co-immunoprecipitation experiments will be performed to determine whether hypoglycosylated proteins are delivered to the STT3B isoform of the OST by lumenal ER chaperones or ER-localized lectins. Inherited defects in the assembly pathway for the dolichol-linked oligosaccharide donor (Dol-PP-OS) for N-linked glycosylation cause congenital disorders of glycosylation (CDG-I). Assembly of Dol-PP-OS will be perturbed in HeLa cells by si-RNA mediated depletion of the ALG6 glucosyltransferase. Simultaneous depletion of ALG6 and STT3B will be used to test whether the STT3B isoform of the OST modifies glycosylation sites that are skipped by the STT3A isoform of the OST in ALG6-deficient cells. Recent reports indicate that mutations in the TUSC3 gene can cause autosomal recessive mental retardation. Si-RNA mediated depletion of TUSC3 will be used to determine whether this OST accessory subunit is required for efficient glycosylation of certain proteins. The yeast and mammalian OST are large (240-270 kD) hetero-oligomeric membrane proteins that may interact directly with the protein translocation channel in the endoplasmic reticulum. Complexes of the OST, the Sec61 protein translocation channel and a translating ribosome will be prepared for cryoelectron microscopy. Structural analysis of these complexes should provide insight into the location of the STT3 active site in the OST complex and the path taken by the nascent polypeptide between the lumenal face of the protein translocation channel and the OST active site.